Vacuum insulated storage tanks for missile use



June 18, 1963 J. H. BECKMAN VACUUM INSULATED STORAGE TANKS FOR MISSILE USE Filed June 30, 1959 2 Sheets-Sheet 1 INVENTOR. JOHN H. BECKMAN 4 TTO NEV June 18, 1963 J. H. BECKMAN 3,094,071

VACUUM INSULATED STORAGE TANKS FOR MISSILE USE Filed June 50, 1959 2 Sheets-Sheet 2 I INVENTOR L JOHN H.BECKMAN United States Patent Q York Filed June 30, 1959, Ser. No. 824,022 9 Claims. (Cl. 102--49) This invention pertains to vacuum insulated tanks for storing low temperature liquids and to their method of fabrication .and operation. In particular, it refers to a double-walled stainless steel tank for storing liquefied cryogenic propellant in a long range missile wherein the tank forms an integral part of the missile outer shell.

Liquid oxygen is in many respects a very satisfactory oxidizer for propulsion of long range missiles. Among the notable advantages to the use of liquid oxygen are the relative ease of handling and cost of production which, for example, is only a [fraction of the cost of other oxidizers such as fuming nitric acid. operationally, liquid oxygen possesses a relatively high density and also provides a comparatively high specific thrust.

- These desirable features, are, however, counter-balanced by two serious disadvantages that greatly detract from the use of liquid oxygen in connection with missile propulsion. One disadvantage resides in an extreme evaporation rate when stored in non-insulated containers, the other disadvantage is that its low temperature (-183 C.) chills and tends to freeze valves and other operative equipment located adjacent to uninsulated containers of liquid oxygen.

In connection with liquid oxygen propelled missiles which are required to be ready for firing at all times, it has heretofore been rather impractical to use oxygen containers taught by the prior art because of the abovenoted evaporation rate and the problems associated with the chilling effects. To satisfactorily cope with these problems, a liquid oxygen missile for stand-by readiness service would require a complex ground supply system for rapidly filling the missile shortly prior to launching. Ordinarily, non-vacuum insulations are not considered practical for missile tanks due to the large insulation volume required to achieve any reasonable degree of effectiveness. Also, the prevalence of heat conductivity along the structures used to connect the liquid oxygen container to the remainder of the vessel, posed a vexing problem. Vacuum insulation with prior art doublewalled containers would normally not be used since the heavy outer vessel required to withstand atmospheric pressure would add undue weight to the missile.

It is therefore an object of the present invention to provide -a container for low temperature fluids having a wall structure possessing excellent insulating properties and also adapted to resist internal and external pressure variations. It is also an object to provide such a container which is characterized by being adaptable to being incorporated as an integral section of a missile body.

A further object is to provide in a vacuum insulation space an improved internal spacing member having a desirable combination of structural strength and high resistivity to conductive heat flow.

Another object is to provide an internal bracing structure for a vacuum panel insulation, adapted to minimize the effects of thermal expansion.

In the drawings:

FIG. 1 is an elevation view in cross-section of a twostage liquid fuel missile employing tanks of the present invention for storing the cryogenic liquid oxidant in both stages,

FIG. 2 is an enlarged cross-sectional view taken at detail 2 in FIG. 1,

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FIG. 3 is an enlarged cross-sectional view of a portion of the missile shown at detail 3 of FIG. l,

FIG. 4 is an enlarged detailed view of a plate member adaptable for supporting items within the interior of the storage tank. This is indicated by detail 4 of FIG. 1,

FIG. 5 is an enlarged cross-sectional view of a stiifening ring adaptable for use in the interior of the storage tank and shown as detail 5 of FIG. 1,

FIG. 6 is a diagrammatic view of typical column and disc spacer arrangement used in the evacuated space between the inner and outer vessels,

FIG. 7 illustrates one embodiment of spacer with end support, and

FIG. 8 is a cross-sectional view of a preferred embodiment of the spacer and end support means.

In brief, the invention contemplates a novel, double wall vacuum insulated container for low temperature fluids adaptable to form an integral portion of a missile body comprising essentially an inner vessel and an outer vessel or shell defining a space therebetween for confining an insulating medium, both the vessel and shell being substantially similar in configuration; and a plurality of low thermally conductive brace members disposed between said inner and outer vessels to maintain a desired spacing therebetween and provide structural support to the container While on the ground and in the air.

The insulating medium between the inner and outer vessel walls in accordance with the invention may consist of -a straight vacuum or the evacuated space may preferably be filled or partially filled with opacified insulation. The term vacuum, as used herein, is intended to apply to sub-atmospheric pressure conditions not substantially greater than 5000 microns of mercury, and preferably below 500 microns of mercury absolute. The designation of opacified insulation, as used herein, refers to a two component insulating system comprising a low heat conductive, radiation permeable material and a radiant heat impervious material which is capable of reducing the passage of infrared radiation rays without significantly increasing the thermal conductivity of the insulating system.

As defined, opacified insulation includes a mixture of finely-divided low-conductive particles which substantially impede heat inleak by conduction and yield to heat passage by radiation, and finely-divided radiant heat impervious bodies having a relatively high thermal conductivity. As more fully described and claimed in copending United States Serial No. 580,897, filed April 26, 1956 in the names of L. C. Matsch and A. W. Francis now Patent No. 2,967,152, the low conductive particles may be selected from the group consisting of silica, perlite, alumina, magnesia, and carbon black, and the radiant heat impervious bodies are preferably either aluminum or copper, although copper paint pigments, alumina paint pigments, magnesium oxide, zinc oxide, iron oxide, titanium dioxide, copper coated mica flakes, carbon black, and graphite alone or in combination with each other would give satisfactory results. Also, these bodies usually in the form of flakes or powder preferably constitute between 1% and of the total weight of the opacified insulation.

The opacified insulation may also take the form of the combination of a low heat conductive material and a multiplicity of spaced radiation-impervious barriers. As more fully described and claimed in copending United States Serial No. 597,947, filed July 16, 1956 in the name of L. C. Matsch now Patent No. 3,007,596, the low heat conductive material may be the previously listed powderous materials or alternatively a fiber insulation which may be produce-d in sheet form.

Examples of the latter include a filamentary glass material such as glass wool and fiber glass, preferably having s3 fiber diameters of less than about 50 microns. Also such fibrous materials preferably have a fiber orientation substantially perpendicular to the direction of heat flow across the insulation space. The spaced radiationimpervious barriers may comprise either a metal, metal oxide or metal coated material, such as aluminum coated plastic film or other radiation reflective or radiation adsorptive material or a suitable combination thereof. Radiation reflective materials comprising thin metal foils are particularly suited in the practice of the present invention, for example, reflective sheets of aluminum foil having a thickness between about 0.2 millimeter and 0.002 millimeter.

Other radiation reflective materials which are susceptible of use in the practice of the invention are tin, silver, gold, copper, cadmium, or other metals. When fiber sheets are used as the low conductive material, they may conveniently serve as a support means for the relatively fragile radiation-impervious sheets. For example, an aluminum foil-fiber glass sheet insulation may be spirally wound around the inner vessel with one end of the insulation wrapping in contact with the inner vessel, and the other end near the outer shell, or in actual contact therewith.

The vacuum insulation systems and especially the opacified vacuum insulations are extremely eflicient for storage of low temperature materials without excessive heat leak. Use of such insulation in conjunction with the present container, enables a missile to remain fully loaded in a stand-by condition for prolonged periods with only minor additions of liquid oxidant from time to time to make up small evaporation losses.

-In FIG. 1 a missile is shown having a first stage section and :a second stage section 11. The first stage 10 consists of a vacuum insulated liquid oxidant tank 12, liquid fuel tank 13 and motor chamber 14 in which combustion takes place to develop the propulsive thrust. The second stage 11 consists of a payload section 15, liquid oxidant tank 16, liquid fuel tank 17 and motor chamber 18. The present invention is concerned specifically with the design and fabrication of cryogenic liquid propellant tanks of the type shown at 12 and 16.

Referring to FIGS. 1 and 2, the liquid oxidant tank 12 consists of an inner vessel 19 and an outer vessel 20 separated by an evacuated space 21. Various metallic materials could be used for vessels 19 and 20. These metals or metal alloys must have high strength at both high and low temperatures, be relatively ductile at temperatures approaching 200 C., have relatively low thermal expansion coefficient, and be weldable in quite thin sections. Among the metals found to comply with these requirements are such as aluminum, aluminum alloys, Monel, Inconel and stainless steel. Aluminum is not considered overly desirable due to the reduced strength characteristics thereof at the high temperatures which must be endured by the outer vessel surface under inflight conditions. Aluminum also possesses a rather high coefficient of expansion, which property tends to foster buckling problems. Further, the various aluminum high strength alloys are not readily adaptable to welding in thin sections for vacuum service.

A preferred metal for the present application is stainless steel, particularly type 304 and 321, one of its most valuable assets being weldability in thickness as small as 0.010 inch. Since weight considerations in missile construction are rather important factors, the combination of maximum strength and minimum weight is most desirable.

In order to reduce the overall weight of the missile and still use relatively dense stainless steel as the material of construction for the liquid fuel tank, the following novel combination of features is employed. First, the walls of both the inner and outer vessels are made quite thin (about 0.010-0030 inch) so that neither is required to withstand by itself the entire pressure differential between inner vessel and ambient atmosphere. To transmit forces acting on one wall to the other wall, insulated spacers 22 are positioned within the evacuated space 21 as shown in FIG. 2. This combination of light wall sections with intermediate spacers, also provides unexpected rigidity for a container having such thin walls both at atmospheric pressure and at ambient conditions found at the high altitude in which such missiles operate.

It is found that the most favorable results are obtained when the spacers 22 are maintained substantially in a state of compression created by the combination of internal pressure within vessel 19 and external atmospheric pressure acting on the outer surface of vessel 20. The amount of compression on any given spacer may vary considerably in operation, for instance, as the ambient pressure decreases with increasing altitudes, the spacers 22 continue to exert force on the outer shell 20 which is now taken up by additional tensile stress in such outer shell. The elastic expansion of the outer shell is thereby coupled with a comparable expansion of the inner shell.

A further advantage of the present invention is obtained by work-hardening the thin vessel walls after fabrication in order to increase their strength. This is accomplished by massive stretching, preferably by hydraulic means, of the separate complete inner vessel and the complete outer vessel. It has been found that a 20% stretching of the walls strengthens the metal to a very high degree by work-hardening. At this strength level, stainless steel has a strength/weight ratio competitive with any weldable aluminum; even aluminum that is only weldable in much thicker sections.

The spacers 22 must be of low thermal conductivity material primarily because the insulation efliciency of vacuum-type insulation would be drastically reduced if said spacers provided a ready path for heat .to be conducted into the inner vessel. Various materials, such as a fabric or paper reinforced organic thermal setting plastic of the phenol-formaldehyde or melamine-formaldehyde types, for example, might be used for the spacers, but porcelain among other ceramic types, is the preferred material due to the high strength/thermal conductivity obtainable at elevated temperature. In a typical missile storage tank operating at about 30-50 p.s.i. internal pressure, these spacers, as shown in FIGS. 7 and 8, will be about 0.060 inch diameter and inch long. In order to increase the distribution of the spacer column stress to the outer vessel 20 and thereby prevent its sagging under normal ambient pressure conditions, it is preferred that a thin slightly conical-shaped disc 23 about /2 inch diameter be positioned on the outer end of each of these spacers. The spacers are held in position with a distance of about one inch between centers as shown in FIG. 6 either by an adhesive bond to the inner vessel or by the opacified insulation or by a combination of both of these methods. Compressive forces also maintain spacer position.

The axial dimension of the spacers 22 is self-limiting. Contingent on the container requirements, each spacer should be sufliciently limited in cross-sectional area, to restrict the path of heat fiow; at the same time, it must be adequate to provide structural rigidity and support to the panel walls. In this respect, various modifications of the spacer could include typical structural shapes as cylindrical, hollow square, hollow tube, or the like. The walls of the spacer should be aligned perpendicular to the adjacent panels in order to minimize any tendency toward buckling under compression.

An important advantage of the bracing means of the present invention resides in the use of end supports such as 32 shown in FIG. 7. These supports, when designed to conform with the contour of the panel surfaces, provide a means for reducing stress concentration on the panel surfaces and rather distribute such stresses over a substantially larger area.

The double-walled vacuum insulated storage tanks of the present invention are fabricated by the following pre ferred method. The inner vessel is fabricated and vacuum leak tested by conventional welding and testing means. Said inner vessel may then be placed inside a form and stretched approximately 20% by the application of internal pressure to within about one inch of the desired vessel .diameter and to approximately the required yield strength of the finished vessel. If opacified l-amell ar insulation of the type described in patent No. 3,007,596 is to be used, this insulation is then applied to the outer surface of the inner vessel after which appropriate holes are cut in the surface of the insulation for the insertion of the insulating spacers. If opacified powder insulation of the type described in Patent No. 2,967,152 is to be used, the insulating spacers are directly bonded to the inner vessel and the insulation is subsequently added to the space provided therefor.

The outer vessel or shell 20 is then separately fabricated with a hoop or ring 24, as shown in FIG. 1, circumferentially attached to the approximate center of the outer vessel. The outer vessel and its circumferential ring are then stretched about 20% to finished size in a manner similar to that used above for the inner vessel. The outer vessel is then cut apart through the outer ring by any convenient means into two sections. These sections are next placed about the inner vessel, after which they are re-welded through the hoop section. Such re-welding does not introduce undesirable stresses in the outer vessel. Internal pressure is then applied to the inner vessel thereby expanding it until the spacers press against the outer vessel. The spacers must remain in compressive contact with the outer shell after such internal pressure is released. If opacified powder insulation is to be used, it is now introduced into the space between the inner and outer vessels. The insulating or annular space 21 is then evacuated, preferably while subjecting the entire unit to an elevated temperature to help drive oft contained gases. The above novel procedure for fabricating a doublewalled missile tank employing work-hardened walls is especially useful and desirable for tanks in the missile first stage. Such tanks are usually too large to be conveniently heat-treated. Cryogenic propellant tanks of upper missile stages, such as tank 16, are considerably smaller. Therefore, the inner vessel and outer shell of such upper stage tanks are preferably individually fabricated in the usual fashion by welding and then are heattreated to relieve welding stresses and to increase strength.

It is preferred that the finished container have -a substantially cylindrical body with hemispherical end sections. This combination provides the best method for withstanding stresses created by internal and ambient pressures, acceleration conditions and aerodynamic considerations peculiar to the operation of missiles.

A fundamental problem with double-walled containers arises when large and changing temperature differences exist between the inner vessel and the outer shell. Such a condition can arise when liquid oxygen, for example, is stored in the inner vessel and the outer shell is at or above room temperature. If standard double-Wall tank construction were used, this problem would cause failure .due to excessive stresses in the walls or due to loss of contact between the walls and their supporting members. This problem is solved in the present invention by so const-ructing the container that the thicknesses of the inner and outer vessel walls are such that they can withstand the operating pressure and absorb stresses created by thermal expansion. These expansion stresses are handled by "a combination of elastic stretching of the outer shell and elastic compression of the inner vessel. In one modification of the present invention, the container is so con structed that the inner and outer vessels will have no significant overall tensile or compressive stresses while under the following conditions; the inner container is at liquid oxygen temperature (-183 -C.), the outer shell is at room temperature (20 0.), and the internal and external pressures are atmospheric. :When the inner container subsequently warms to room temperature, expansions can be taken care of by a combination of elastic stretching of the outer shell, elastic compression of the inner vessel and novel use of elastic controlled buckling of the inner shell between the spacer columns. The amount of such buckling is controlled by choice of spacer size and distance between columns. This elastic controlled buckling can also be used for containers having operating conditions ditferent from those described above.

To avoid excessive stresses being set up due to elastic buckling of the inner vessel, one modification of spacer 22, as shown in FIG. 7, has a bar-like end support 32 positioned against the out-er surface of the inner shell. The spacer also has a conical shaped disc-like support 23 which contacts the outer shell. The long axis of the bar 32 should extend circumferentially. This bar provides a maximum longitudinal length of the inner shell between supports in order to allow the required buckling deflection to take place with a minimum stress. The bar preferably has a slightly curved surface 33 parallel to such longitudinal axis to allow free bending of the inner shell. In practice, it may be alternatively preferable to have a slight circumferential corrugation 35 of the inner vessel that would receive the end of the spacer. This is shown in FIG. 8. When the insulation space is not completely filled with opacified insulation, heat leak along spacer column 22 can be minimized by wrapping such column with Fiberglas insulation 36. Additional support for the inner vessel against sagging and bending can be provided by stiffening rings exemplified by ring 34 shown in RIG. 1 and in more detail in FIG. 5. I

The above discussion concerning thermal expansion of the inner Vessel and methods of accomplishing controlled buckling of such vessel is directed primarily at a tank fabricated from stainless steel. Such conditions may alternatively be overcome by the use of Invar as the material for the inner vessel. This type of metal alloy, when subjected to severe temperature changes, exhibits very little contraction or expansion tendencies.

The missile oxidant tank 12 defines the upper portion of the missile first stage and is connected to the rearwardly disposed fuel tank 13 by means shown in FIG. 3. As an indication of the magnitude of the invention, tank 12 could be about 10 feet in outer diameter and about 3 0 feet long.

In promoting the combustion process of the missile, suitable pressure to force oxidant and fuel through lines 25 and 26 and into the motor chamber 14, may be obtained in several ways. For example, vaporization of the contained liquid can be utilized to supply the desired pressure, but this wastes oxidant which would otherwise be used in the combustion process. Referring to FIG. 1, a preferred method is to incorporate a separate pressure source 27 containing pressurized gas, such as helium, within the oxidant storage tank. It may then be introduced in desired amounts through line 28 and into the oxidant tank and connecting line 29, thence into the fuel tank. Such a pressure source may be suitably supported by means such as the struts 30 attached to the inner vessel by support 31 as shown in FIGS. 1 and 4.

Another problem which is encountered with missile operation and which can become serious for double walled vacuum insulated containers is specifically that of temperature rise in the outer wall of the tank outer vessel. For instance, at high speeds, skin friction will cause the missile outer surfaces to heat up to temperatures as high as 1000 F. and consequently expand. When spacers are used between relatively thin walls, as in the present invention, the outer vessel expansion causes said spacers to lose contact with the outer vessel and thereby reduce the structural rigidity of the missile formerly maintained by the combination of both vessels. This problem is overcome by the present invention by several means.

One method of maintaining an adequately low external skin temperature would be to percolate liquid oxygen up through the insulation space after takeoff of the missile. If the liquid oxygen were released after takeoff by a fuse plug at such time as the skin temperature reached its allowable limit, the altitude should be great enough to avoid external icing. The very thin (about inch) insulation space should provide excellent percolation in that the mean fluid density in the insulation space should be only a fraction that of pure liquid. Also a significant portion of the upper end of the missile container should be adequately temperature equalized by the flowing cold gas.

Utilizing this system, the liquid oxygen should be released through .a pressure reducing valve into the lower portion of the insulation space. The rate of flow could be controlled by any appropriate means such as a thermostatic switch at the top of the tank or by a strain indicator connecting the inner and outer vessels. In such event, the gas would vent overboard at the top of the tank. This method possesses the further advantage of keeping the remaining liquid in the inner vessel in a subcooled state due to the fact that the boiling pressure in the insulation space is lower than the pressure in the main body of liquid. With this arrangement, even the last portions of oxygen to leave the tank would be subcooled :and would be more likely to pass through line 25 and any associated pump withoutvapor bin-ding and cessation of flow. In this fashion virtually all of the oxidant can be removed for use in combustion with fuel. This reduces the burnout weight of the missile. The main disadvantage of this method of reducing outer skin temperature is the consumption of oxygen. This oxygen consumption, however, affects primarily the takeoff weight and is not nearly as detrimental to overall final weight of the missile as would be an equivalent weight of structure.

The preferred method of solving the in-flight expansion problem of the outer shell is to introduce helium from the tank pressurizing source 27 into the insulation space 21 sometime after takeoff. This could also be done through the use of a fusible metal plug so located as to be melted by rising skin temperature. In this event, the helium pressure should be reduced to a pressure intermediate that of the tank operating pressure and the external pressure by means of .a regulator.

The high thermal conductivity of helium can be counted on to remove some of the heat from the outer shell and to maintain the shell of the inner vessel above the oxygen liquid level at approximately the same temperature as the adjacent portion of the outer vessel. As the oxidant is consumed in combustion, the inner vessel length will gradually increase and approach the temperature of the outer vessel. The spacers will then 'remain in contact with both inner and outer vessels to maintain the desired structural rigidity.

Another advantage of the present invention for use with low temperature liquid oxidants is the minimization of frost or ice formation on the external surface of a missile as it rises through clouds or other moisture-containing atmospheres. Such frost or ice would usually form in copious quantities when an uninsulated missile tank was used with liquid oxygen as the oxidant; such formations would of course undesirably increase the missile in-flight weight. This frost formation is'found to be virtually negligible when the present invention is used.

A vacuum necessaiy to maintain the missile in standby-ready condition for years could be easily maintained by a vacuum pump connected to the evacuated space 21. Substantially leak-free welds can easily be obtained with stainless steel. Therefore, this potential pumping is considered to be a very minor item.

The over-all in-flight weight of a missile tank such as illustrated by item 12 of FIG. 1 using aluminum in single wall construction, as taught by the prior art, is substantially the same as the weight of a thin double-walled stainless steel vessel constructed according to the present invention. The latter construction, though, possesses the overwhelming advantage of enabling rockets and missiles employing a liquid oxygen oxidant to be maintained in stand-by readiness for extended periods of time. It also precludes the usual problems associated with rapidly fillin the missile tanks shortly prior to launching.

While the foregoing discussion of the invention has been directed primarily to the construction of liquid oxygen tanks for the missile first stage, it is understandable that such construetion could readily be utilized equally as well for upper stages such as 11, and also for the storage of other cryogenic propellants such as liquid fluorine and liquid hydrogen.

What is claimed is:

l. A cryogenic liquid storage container for missile use comprising: an inner vessel, an outer shell surrounding and outwardly spaced from said inner vessel defining an evacuable insulating space therebetween; a plurality of low thermally conductive brace members positioned a substantially uniform lateral distance apart in said insulating space; said inner vessel and outer shell having a wall thickness sufliciently thin such that neither is capable of withstanding by itself the entire pressure ditferential between said inner vessel and the ambient atmosphere; and said brace members, said inner vessel and said outer shell being cooperatively constructed and arranged to maintain said brace members in a state of compression, to elastically compress said inner vessel such that the inner vessel walls elastically buckle between said brace members, and to elastically stretch said outer shell.

2. A double-Walled container substantially as described in claim 1 wherein the inner vessel and outer shell are constructed of stainless steel.

3. A double-walled container substantially as described in claim 1 wherein the low thermally conductive brace members are a non-metallic compression resistant material.

4. A double-walled container substantially as described in claim 1 wherein the brace member is porcelain.

5. A double-walled container substantially as described in claim 1 wherein the brace members each have a plate afiixed to at least one end thereof, said plate disposed substantially perpendicular to said elongated section.

6. A double-walled container substantially as described in claim 1 wherein the insulating space intermediate said vessel and said shell is evacuated and is provided with an opaeified insulating material.

7. A double-walled container substantially as described in claim 1 wherein the inner vessel and the outer shell each being formed of metal between 0.010 and 0.030 inch thick, an opacified insulating material substantially filling the insulating space between inner vessel and outer shell.

8. A double-walled container substantially as described in claim 7 wherein the walls of the inner vessel and outer shell are formed of thin work-hardened metal.

9. A double-walled container substantially as described in claim 1 wherein the spacer member comprises a reinforced organic thermosetting resin.

References Cited in the file of this patent UNITED STATES PATENTS 2,217,649 Goddard Oct. 8, 1940 2,513,749 Schilling July 4, 1950 2,722,336 Wexler et al. Nov. 1, 1955 2,776,069 Zimmermann Jan. 1, 1957 2,874,539 Fox Feb. 24, 1959 2,874,865 Canty et al. Feb. 24, 1959 2,902,822 McKiernan Sept. 8, 1959 2,972,225 Cumming et a1 Feb. 21, 1961 

1. A CRYOGENIC LIQUID STORAGE CONTAINER FOR MISSILE USE COMPRISING: AN INNER VESSEL, AN OUTER SHELL SURROUNDING AND OUTWARDLY SPACED FROM SAID INNER VESSEL DEFINING AN EVACUABLE INSULATING SPACE THEREBETWEEN; A PLURALITY OF LOW THERMALLY CONDUCTIVE BRACE MEMBERS POSITIONED A SUBSTANTIALLY UNIFORM LATERAL DISTANCE APART IN SAID INSULATING SPACE; SAID INNER VESSEL AND OUTER SHELL HAVING A WALL THICKNESS SUFFICIENTLY THIN SUCH THAT NEITHER IS CAPABLE OF WITHSTANDING BY ITSELF THE ENTIRE PRESSURE DIFFERENTIAL BETWEEN SAID INNER VESSEL AND THE AMBIENT ATMOSPHERE; AND SAID BRACE MEMBERS, SAID INNER VESSEL AND SAID OUTER SHELL BEING COOPERATIVELY CONSTRUCTED AND ARRANGED TO MAINTAIN SAID BRACE MEMBERS IN A STATE OF COMPRESSION, TO ELASTICALLY COMPRESS SAID INNER VESSEL SUCH THAT THE INNER VESSEL WALLS ELASTICALLY BUCKLE BETWEEN SAID BRACE MEMBERS, AND TO ELASTICALLY STRETCH SAID OUTER SHELL. 